In the kraft process, high
alkalinity and temperature convert the esters and carboxylic acids in
rosin into soluble sodium
soaps of
lignin, rosin, and
fatty acids. The spent cooking liquor is called weak
black liquor and is about 15% dry content. The black liquor is concentrated in a
multiple effect evaporator and after the first stage the black liquor is about 20–30%. At this stage it is called intermediate liquor. Normally the soaps start to
float in the storage tank for the weak or intermediate liquors and are
skimmed off and collected. A good soap skimming operation reduces the soap content of the black liquor down to 0.2–0.4% w/w of the dry residue. The collected soap is called
raw rosin soap or
rosinate. The raw rosin soap is then allowed to
settle or is
centrifuged to release as much as possible of the entrained black liquor. The soap goes then to the
acidulator where it is heated and acidified with
sulfuric acid to produce
crude tall oil (CTO). The soap skimming and acidulator operation can be improved by addition of
flocculants. A flocculant will shorten the separation time and give a cleaner soap with lower viscosity. This makes the acidulator run more smoothly as well. Most pines give a soap yield of 5–25 kg/ton pulp, while
Scots pine gives 20–50 kg/ton. Scots pine grown in northern Scandinavia give a yield of even more than 50 kg/ton. Globally about 2 mill ton/year of CTO are refined. ==Composition==